The proposed research will significantly expand our understanding of muscle function by studying a variety of locomotory behaviors in horses carrying a portable behaviors in horses carrying a portable sonomicrometry system. Currently, the commonly accepted "Force Hypothesis" that relates locomotory mechanics to the rate of energy consumption states that the muscles supporting the body during locomotion function isometrically, generating force without shortening. The theory is that this permits the tendons to which the muscles are attached to change length, alternatively storing and releasing energy. An important consequence of this proposed function is that the muscles produce no power (power= force x muscle shortening velocity). The research we propose is based on the simple assumption that the observable diversity in muscle anatomy is reflected in a broader range of muscle functions. We hypothesize that hip and knee extensors with long fibers and short tendons produce some power during running while ankle extensors with short fibers and long tendons conform to the Force Hypothesis and function isometrically This is an attractive hypothesis because it promises to explain several published observations indicating that some power is required during locomotion. We propose to study two muscles, the gluteus medius, a hip extensor, and the vastus lateralis, a knee extensor. We will directly measure changes in muscle length with sonomicrometry and estimate the forces generated by the muscles with the use of a force platform and kinetic analysis. These measurements will be made on horses that have been trained to perform different behaviors requiring different amounts of power: running at a steady speed on a treadmill; running up an incline; accelerating from a stationary position to a gallop; or jumping over a 50 cm barrier. Several of these studies are made possible by our portable data acquisition system that the horses carry while performing these behaviors. Preliminary data indicate that the vastus shortens 50% more than has been reported for the gastrocnemius during running at a constant speed on a treadmill, and that it dramatically increases its shortening during acceleration and jumping.